Vehicles recently developed often have a function to follow preceding vehicles, which is one of a variety of functions to reduce driving loads on drivers. This following function is referred to as adaptive cruise control (ACC). In a vehicle equipped with the ACC, an ACC system including a CPU automatically performs drive control (accelerating and braking operations) of the vehicle such that an inter-vehicle distance D between the vehicle and a preceding vehicle coincides with a target inter-vehicle distance Dtr. Unfortunately, the process of drive control and the timing to execute drive control vary between drivers, and some drivers may feel awkwardness or discomfort to the drive control executed by the ACC system.
Japanese Unexamined Patent Application Publication No. 2004-122823 (claim 6, paragraphs [0030] and [0038], FIGS. 8 to 10) presents the results of research showing that drivers feel an appropriate inter-vehicle distance D as short when the number of lanes N is small. Based on these results, the control apparatus according to Japanese Unexamined Patent Application Publication No. 2004-122823 increases the target inter-vehicle distance Dtr as the number of lanes N is reduced. Specifically, the control apparatus according to Japanese Unexamined Patent Application Publication No. 2004-122823 acquires information on the number of lanes on a road using a navigation system. Drive control is executed such that a first target inter-vehicle distance Dtr 1 is attained, if a road has three or more lanes in each direction. If the road has two lanes in each direction, the first target inter-vehicle distance Dtr 1 is multiplied by a first gain (>1) according to the vehicle speed to determine a second target inter-vehicle distance Dtr 2, and drive control is executed such that the second target inter-vehicle distance Dtr 2 is attained. If the road has one lane in each direction, the first target inter-vehicle distance Dtr is multiplied by a second gain according to the vehicle speed (>first gain) to determine a third target inter-vehicle distance Dtr 3, and drive control is executed such that the third target inter-vehicle distance Dtr 3 is attained.
The control apparatus according to Japanese Unexamined Patent Application Publication No. 2004-122823 enables control of the inter-vehicle distance according to the number of lanes N without giving awkwardness or discomfort to the drivers. In contrast, the control apparatus according to Japanese Unexamined Patent Application Publication No. 2004-122823 executes control of acceleration or deceleration in a fixed manner irrespective of the number of lanes N. Such control of acceleration or deceleration may cause giving awkwardness or discomfort to the drivers.
For example, if the vehicle is driving on a multiple-lane road having multiple lanes in each direction (N≥2), an inter-vehicle distance D between the vehicle and the preceding vehicle larger than the target inter-vehicle distance Dtr readily allows cut-in by another vehicle in front of the own vehicle. Drivers who do not want such cut-in want quick return of the inter-vehicle distance D to the target inter-vehicle distance Dtr if the inter-vehicle distance D with the preceding vehicle is larger than the target inter-vehicle distance Dtr during driving on a multiple-lane road. In contrast, cut-in by another vehicle does not occur when the vehicle is driving on a single-lane road having a single lane in each direction (N=1). Accordingly, the drivers desire control of acceleration or deceleration with sufficient allowance.